<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Title</title>
</head>
<body>

<script>
  for (var i = 1; i <= 3; i++) {
    console.log(i)
  }
  // 发现此时 i 在在全局范围内被修改成了4
  console.log(i)

  for (let j = 1; j <= 3, j++) {
    console.log(j)
  }
  console.log(j)
</script>
</body>
</html>